In the case of driveable and hence mobile working machines, the particular constraints and demands placed on the structural design of the hydraulic devices have resulted in the independent category of mobile hydraulics being developed in parallel to stationary hydraulics, and the invention relates to the technical field of mobile hydraulics. In hydraulic drives for controlling a hydraulic cylinder or hydraulic motor, the drive movement normally occurs with pressure and throughflow generated in a pump unit against the load forces acting on the cylinder from outside counter to the direction of movement (positive load forces). However, it is also possible in the course of movement for negative load forces to occur in the direction of movement—such as during lowering of lifted loads, a braking of moved masses or load direction reversal—which result in undesired leading and uncontrolled lowering of the cylinder. In addition to the uncontrolled movement, a negative pressure with cavitation would occur on the cylinder side driven by the pump throughflow, with the result that the hydraulic system may be damaged. In order to control the working cylinders in mobile hydraulic machines, use is made of 6/3-port directional control valves of piston slide valve type with a proportional throttling function which are designed specifically for use in mobile hydraulics and which, upon activation, throttle in a proportionally controlled manner both the oil inflow from the pump to the working cylinder and the oil outflow from the working cylinder to the tank. The main working movements—generally during the extension of the cylinder—occur with positive force loadings, wherein the load acts in a pushing manner counter to the desired direction of movement of the consumer. However, negative force loadings can also occur in both directions of movement, wherein the load acts in a pulling manner in the same direction as the desired direction of movement, such as, for example, during the lowering of loads, braking of large moved masses and load change of externally acting forces. As a consequence, the volumetric flow flowing from the cylinder to the tank must be throttled in order to prevent undesired acceleration and uncontrolled movement of the cylinder, and it is known to provide valves having a lowering braking function for this purpose. In mobile hydraulics, use is made of complex mobile control blocks having a plurality of 6/3-port directional control valves with all the required additional functions, including the throughflow distribution to the connected cylinders from a delivery pump.
Excavator booms and other working manipulators, such as shovels, buckets or sliding ploughs, within the sector of mobile working machines are nowadays predominantly controlled by the operator by means of hand lever pre-control devices (joysticks). When problematic operating states occur, which may be caused for example by changing loads or particularly quick or slow movements, the operator must then perform a corresponding actuating signal correction to maintain the desired setpoint movements, something which requires appropriate training and experience. With regard to boom and dipper cylinder control of a shovel excavator, there is obtained a separate function whereby, after the extension operation, the lowering during the retraction of the cylinders is intended to take place through self-weight without pump inflow. This function is referred to below as “floating”. For this purpose, the piston side and rod side of the working cylinder are bypass-connected or short-circuited. The oil displaced from the piston side through the force of the weight flows, in order to replenish the oil volume sucked away, partially to the rod side and the residual quantity flows to the tank. The lowering speed is electrohydraulically proportionally controlled by a throttling bypass valve in a variable remote-controllable manner. The residual quantity flowing to the tank flows via a pre-stressing return valve which pre-stresses the pressure in the cylinder connection to such an extent that no cavitation can occur in the cylinder through flow losses in the cylinder line. These valves which are required for the lowering in bypass mode through self-weight must additionally be installed in the main flow with corresponding throughflow capacity between the mobile control block and cylinder. Since the mobile hydraulics used to date produce a throughflow in the part-load range via a bypass, there occur considerable hydraulic energy losses which considerably reduce the efficiency of the drive and require a large cooling capacity of the hydraulic system. This loss effect occurs particularly when braking negative load forces in the direction of movement since, in order to throttle the throughflow flowing back from the cylinder, the hitherto used valve units with valve slides have to be actuated in the closing direction always in the fine-control range with control edge undercutting. These hydraulic energy losses caused by the valve control principle come increasingly to the fore as a disadvantage as the overall size and drive power of the mobile working machine increases.
In particular in the case of large mobile machines and large-area excavators as are used, for example, in open-cast mining, given the high loads to be controlled, the required throughflow quantities and throughflow rates of far above 1000 L/min (264 gal/min), and the aforementioned disadvantages, mobile machines with a cable control are usually used.